1. Field of the Invention
This invention relates broadly to prostheses. More particularly, this invention relates to eye prostheses for improving vision and methods of use of such prostheses for the treatment of disorders of the eye.
2. State of the Art
The human eye generally comprises a cornea, an iris, a ciliary body (muscle), a capsular bag having an anterior wall and a posterior wall, and a natural crystalline lens contained within the walls of the capsular bag. The capsular bag is connected to the ciliary body by means of a plurality of zonules which are strands or fibers. The ciliary body surrounds the capsular bag and lens, defining an open space, the diameter of which depends upon the state (relaxed or contracted) of the ciliary body.
When the ciliary body relaxes, the diameter of the opening increases, and the zonules are pulled taut and exert a tensile force on the anterior and posterior walls of the capsular bag, tending to flatten it. As a consequence, the lens is also flattened, thereby undergoing a decrease in focusing power. This is the condition for normal distance viewing. Thus, the emmetropic human eye is naturally focused on distant objects.
Through a process termed accomodation, the human eye can increase its focusing power and bring into focus objects at near. Accomodation is enabled by a changes in the anatomy, particularly including the relationship between the lens, zonules and ciliary body. According to the generally accepted Helmholtz theory of accomodation, when the ciliary body contracts, the diameter of the opening is decreased thereby causing a compensatory relaxation of the zonules. This in turn removes or decreases the tension on the capsular bag, and allows the lens to assume a more rounded or spherical shape. This rounded shape increases the focal power of the lens such that the lens focuses on objects at near.
As such, the process of accommodation is made more efficient by the interplay between stresses in the ciliary body and the lens. When the ciliary body relaxes and reduces its internal stress, there is a compensatory transfer of this stress into the body of the lens, which is then stretched away from its globular relaxed state into a more stressed elongated conformation for distance viewing. The opposite happens as accommodation occurs for near vision, where the stress is transferred from the elongated lens into the contracted ciliary body.
As humans age, there is a general loss of ability to accommodate, termed “presbyopia”, which eventually leaves the eye unable to focus on near objects. According to conventional theory, this loss in ability to focus on near objects is a consequence of a loss in elasticity of the lens capsule and/or sclerosis of the lens with age. Consequently, even though the radial tension of the zonules is relaxed by contraction of the ciliary bodies, the lens fails to assume a greater curvature.
Hideharu Fukasaku, M.D., has developed a procedure for the treatment of presbyopia, described in Anterior Ciliary Sclerotomy with Silicone Expansion Plug Implantation (ACS-SEP), Handout for A.S.C.R.S. Course No. 1107: Presbyopia: Is Surgery Able to Compensate for Loss of Accommodation? (Jun. 1, 2002). Referring to prior art FIGS. 1 and 2, in the procedure, deep radial incisions (limbal peritomies) 10 are made in four oblique quadrants of the sclera 12 over the ciliary body and between the insertions of the four main extraocular muscles. The incisions 10 are each approximately 3 mm in length and at 90% depth. Each incision 10 is started at about 1 mm from the corneal surgical limbus 14 and extends 3 mm radially from that location. Lateral pockets (not shown) are preferably defined at the full depth of the incisions, and each pocket is spread with a forceps. A small hand-cut length of a silicone rod (plug) 16 is implanted into each incision. The conjunctiva is then draped over the incision sites, and the sides of each incision are secured with an absorbable suture 18. The purpose of sewing the silicone implant into the incision is to maintain the effect of making the incision. In fact, making the incision alone tends to help people read at near distance, but this effect diminishes within several months. The implant 16 appears to be a barrier that prevents the scleral tissue from healing, reapproximating and closing the incision, thus maintaining the effect of the incisions.
One theory suggesting why the Fukasaku method increases near distance vision, and thus provides a treatment for presbyopia, is that making the incisions lengthens the circumference of the sclera across the incision, thus providing additional space inside the eye for the ciliary body to contract and cause more effect upon the crystalline lens. If this theory is correct, sewing the silicone implants into their respective incisions increases the effect of creating room for the ciliary body to expand.
An alternate theory suggests that sewing the silicone implants into the incisions places additional pressure on the internal structures of the eye. This causes an increased pressure on the vitreous body to push on the crystalline lens, thus moving it forward which increases the optical effect of the ciliary body. That is, the internal dynamics of the eye are sufficiently altered to at least partially reverse the effects of presbyopia.
While the Fukasaku procedure has merit, the hand-cut silicone implants require physician time to prepare and, due to their hand-cut nature, are inconsistent in length and even shape. Moreover, the shape of the implants is not ideal for secure implantation in the sclera and for providing the desired stress on the sclera.
Spencer Thornton, M.D., and Jim Hayes, M.D., have developed a pre-manufactured titanium implant that can be used in place of Fukasaku's hand-cut silicone plugs. The uniform implants have an inverted T-shape in which the laterally extending portions are intended to seat within the lateral pockets of the incision described by Fukasaku. Initial results suggest that such implants permit more uniform results when using the Fukasaku methodology. However, the Thornton implants are not ideally shaped for scleral implantation, and may be expelled by the sclera.
Ronald Schacher, M.D., has also proposed several other concepts for scleral implants. See, for example, U.S. Pat. Nos. 5,489,299, 6,197,056, and 6,299,640, which are incorporated by reference herein in their entireties. While Schacher proposes a theory as to why presbyopia correction occurs which is contrary to the conventional Helmholtz theory, it appears the Schacher implants may provide the same function as the others (regardless of which theory is correct). Nevertheless, the Schacher implants are subject to the same limitations as other premanufactured scleral implants.